1.5 Amp Negative Adjustable Regulator
8
Application Information
Output Voltage
The output voltage is determined by two external resistors, R
1
and R
2
V
IN
SGR137A
ADJ
V
OUT
I
ADJ
V
REF
-V
IN
R2
+
C
2
-V
OUT
+
5 µF
C
1
+
10 µF
R1
C
3
1 µF
Figure 9 · Output Voltage
The exact formula for the output voltage is:
Where: V
REF
=Reference Voltage and I
ADJ
= Adjustment Pin Current. In most applications, the second term
is small enough to be ignored, typically about 0.5% of V
OUT
. In more critical applications, the exact formula
should be used, with I
ADJ
equal to 65 µA. Solving for R
2
yields:
Smaller values of R
1
and R
2
reduce the influence of I
ADJ
on the output voltage, but the no-load current drain
on the regulator is increased. Typical values for R
1
are between 100 Ω and 300 Ω, giving 12.5mA and
4.2mA no-load current. There is an additional consideration in selecting R
1
the minimum load current
specification of the regulator. The operating current of the SGR137A flows from input to output. If this
current is not absorbed by the load, the output of the regulator rises above the regulated value. The current
drawn by R
1
and R
2
is normally high enough to absorb the current, but care must be taken in no–load
situations where R
1
and R
2
have high values. The maximum value for the operating current, which must be
absorbed, is 5mA for the SGR137A. If input and output voltage differential is less than 10V, the operating
current that must be absorbed drops to 3mA.
Examples:
1. A precision 10V regulator to supply up to 1 Amp load current.
a. Select R
1
= 100Ω to minimize effect of I
ADJ
b. Calculate
A 15 V regulator to run off batteries and supply 50mA. V
IN
MAX = 25V
c. To minimize battery drain, select R1 as high as possible
use 404Ω, 1%
